The stimulation of cholinergic preganglionic nerve fibers resulted in an increase in cyclic guanosine monophosphate (cyclic GMP) and cyclic adenosine monophosphate (cyclic AMP) in the sympathetic ganglia of the bullfrog. When the release of synaptic transmitter was prevented by a high-magnesium/low-calcium Ringer's solution, stimulation of preganglionic nerve fibers did not increase either cyclic GMP or cyclic AMP content. Atropine, a muscarinic antagonist, blocked the increase in cyclic GMP caused by preganglionic stimulation, but not the increase in cyclic AMP. The data indicate that the increases in both cyclic GMP and cyclic AMP are associated with synaptic transmission and are consistent with the possibility that cyclic GMP may be involved in the postsynaptic action of acetylcholine at muscarinic cholinergic synapses.